1 . Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a backlight driving apparatus of a liquid crystal display device that may prevent a shift of a virtual ground point of a u-shaped lamp.
2 . Discussion of the Related Art
Generally, liquid crystal display devices (hereinafter, referred to as an LCD) are being more widely used due to its characteristics of light weight, thinness, low power consumption, etc. Because of these characteristics, the liquid crystal display device is used in office automation equipment, audio/video equipment, etc. The liquid crystal display device controls the amount of light transmitted through a liquid crystal layer in accordance with a video signal applied to a plurality of control switches that are arranged in a matrix, thereby displaying a desired picture on a screen.
The liquid crystal display device is not a self luminous display device, thus it requires a separate light source such as a backlight. Such a backlight may use a cold cathode fluorescent tube (hereinafter, referred to as a CCFL) as a light source.
The CCFL is a light source tube which uses an electron emission phenomenon generated by a strong electric field applied to the surface of a cold cathode, and it is easy to be made with low heat generation, high brightness, long life span, full colorization, etc. The CCFL includes a glass tube where a fluorescent material is spread on the inner wall thereof and an electrode that is stuck to both ends of the glass tube. The glass tube is sealed off with a rare gas like argon and a fixed quantity of mercury.
If a voltage is applied between electrodes of both ends of the glass tube, electrons are emitted to ionize the gas within the glass tube. An electromagnetic discharge with a wavelength of 253.7 nm results that starts by the ionization and recombination of the electron and the ion, and this wavelength excites the mercury to generate an ultraviolet light with a wavelength of 254 nm. The ultraviolet light excites the fluorescent material spread within the inner wall of the CCFL to emit visible light.
The backlight of the liquid crystal display device uses an inverter to produce a high voltage AC power from a low voltage DC power.
Referring to FIG. 1, the related art backlight driving apparatus of the liquid crystal display device includes: a U-shaped lamp 2; an inverter 4 including transformers 6A, 6B each of which corresponds to a (+) electrode and a (−) electrode of the U-shaped lamp 2; and a connector 8 for connecting the (+)(−) electrodes of the U-shaped lamp 2 with the transformers 6A, 6B respectively.
The backlight driving apparatus of the liquid crystal display device is described in detail in conjunction with FIG. 2. The inverter 4 includes first and second switching parts 32A, 32B that generate the AC voltage in accordance with a control signal of a controller 10; first and second transformers 6A, 6B that are connected to the first and second switching parts 32A, 32B respectively for boosting the generated AC voltage to supply to the U-shaped lamp 2; a voltage detector 20 that detects a voltage of the first and second transformers 6A, 6B and transmits the detected value to the controller 10; and the controller 10 that receives the voltage detected from the voltage detector 20 to control the first and second switching parts 32A, 32B.
The first transformer 6A includes a primary winding 34A; an auxiliary winding 36A; and a secondary winding 38A that generates an AC high voltage by being induced by an AC voltage that is generated in the primary winding 34A by the switching of the first switching part 32A.
The second transformer 6B includes a primary winding 34B; an auxiliary winding 36B; and a secondary winding 38B that generates an AC high voltage by being induced by an AC voltage that is generated in the primary winding 34B by the switching of the second switching part 32B.
Herein, a high end of the primary winding 34A of the first transformer 6A and a low end of the primary winding 34B of the second transformer 6B are connected to each other, and a low end of the primary winding 34A of the first transformer 6A and a high end of the primary winding 34B of the second transformer 6B are connected to each other.
The voltage detector 20 detects the AC high voltage, which is induced at the secondary windings 38A, 38B of the first and second transformers, at the low ends of the secondary windings 38A, 38B, thereby generating a feedback voltage. Detection resistors RA, RB each connected to the low ends of the secondary windings 38A, 38B enable the voltage detector 20 to detect the feedback voltage.
The controller 10 receives the feedback voltage F/B generated from the voltage detector 20 to control the first and second switching parts 32A, 32B.
In the case where the feedback voltage F/B is larger than a pre-set reference value, the controller 10 controls a duty ratio of first and second switching parts 32A, 32B, thereby making a voltage lower than the reference voltage transmitted to the U-shaped lamp 2.
On the contrary, in the case where the feedback voltage F/B is smaller than the pre-set reference value, the controller 10 controls the duty ratio of the first and second switching parts 32A, 32B, thereby making the voltage higher than the reference voltage transmitted to the U-shaped lamp 2.
In this way, the backlight driving apparatus of the liquid crystal display device of the related art outputs voltages for their phase difference to be 180°, from the secondary windings of the first and second transformers 36A, 36B of the inverter 4, thereby driving the U-shaped lamp 2.
Theoretically, when a voltage with a 180° phase difference is applied to the U-shaped lamp 2 , a virtual ground point of the applied voltage is formed at center of a bent part of the U-shaped lamp 2 , as shown in FIG. 3A.
However, a difference is generated in the size of the voltages V1, V2 of the secondary windings of the transformers due to variations in the inverter parts (transformer, resistor, etc). Also, a DC noise component flows in from the outside, thus a virtual ground point moves in a number {circle around (1)} or {circle around (2)} direction.
In this way, if the location of the virtual ground point is changed, the distance between both electrodes (+),(−) of the U-shaped lamp and the virtual ground point is not identical, but there is a difference. For example, in the case where the virtual ground point is made to move in the number {circle around (1)} direction, the distance between the (−) electrode and the virtual ground point is larger than the distance between the (+) electrode than the virtual ground point, as shown in FIG. 3B.
Accordingly, when a voltage of the same size, e.g., 900V, is applied, a part of the U-shaped lamp to which (−)900V is applied has a relatively lower brightness than a part of the U-shaped lamp to which (+)900V is applied. Further, a phenomenon occurs where mercury is clustered because the part of the U-shaped lamp to which (−)900V is applied becomes relatively colder than the part of the U-shaped lamp to which (+)900V is applied, thus a problem results in that the life span of the lamp is lowered.